The present invention concerns fine-diameter glass fiber for use in making filters such as HEPA and UPLA filters and low-boron, high-barium glass compositions for making the same.
The glass fibers used in high-efficiency particulate air (xe2x80x9cHEPAxe2x80x9d) filtration media and ultra-low penetration air (xe2x80x9cULPAxe2x80x9d) filtration media typically contain significant amounts of boron, e.g., from about 8 to 11 weight percent of boric oxide. The compositions of some typical glass fibers used in HEPA filters are listed in Table A. The most common method for manufacturing fine-diameter glass fibers, like those of Table A, is xe2x80x9cflame attenuation.xe2x80x9d Flame attenuation involves subjecting strands of glass filaments to high-velocity gaseous blasts from a combustion burner. Another typical method, known as the rotary or CAT process, utilizes centrifugal or rotary forces where the hot molten glass is forced through small orifices in the outer wall of a metal, rotary spinner.
Conventional HEPA and ULPA glass fiber filters, utilizing compositions like those listed in Table A, contain glass microfibers with diameters from about 0.1 xcexcm to about 3.0 xcexcm. Boric oxide is included in the typical glass composition because it results in glass compositions having relatively low viscosity/temperature relationships. The low viscosity/temperature relationships allow the glass composition to pass easily through the orifices in the walls of the spinners so that relatively fine-diameter fibers are produced. The presence of boric acid also reduces corrosion and erosion of the spinner metal. Furthermore, boric acid helps to prevent deterioration or degradation of the glass fiber in aqueous or highly humid environments.
Recent demands for HEPA and ULPA filters used in xe2x80x9cclean roomxe2x80x9d environments for pharmaceutical, biomedical, semiconductor, and related applications, however, require the filters be substantially free of boron. Conventional HEPA and ULPA filters, like those listed in Table A, contain levels of boron that typically cause unacceptable air contamination with boron and thus are not suitable for clean room use. For example, during typical hydrofluoric (HF) acid etch processing of a micro-electronic wafer chip, the HF can combine with boron from the HEPA glass fiber filter to produce boron hexafluoride. The presence of boron hexafluoride has been found to be detrimental to production of high-density memory integrated circuits. Consequently, there is a need in the semi-conductor industry for HEPA filters free of boron but having viscosities that allow fine fiber production via conventional fiber manufacturing methods. That is, the glass composition for making such fibers must be capable of being drawn into fine diameter fibers by flame attenuation, rotary spinning, and CAT processes and must provide a filter that is resistant to deterioration even in high-humidity environments.
Low-boron, high-barium glass compositions and resultant fine-diameter glass fibers used to form HEPA and ULPA glass fiber filters for clean room use, are provided. The compositions and resulting glass fibers preferably comprise a low boric oxide (B2O3) concentration, i.e., less than about 1 weight percent, a relatively high concentration of barium, such as from about 5.5 to about 18 weight percent barium oxide (BaO), and a concentration of alkali ranging from about 10 to about 14.5 weight percent. Alumina (Al2O3) is preferably present in the glass compositions and the resulting glass fibers in a range of from about 4 weight percent to about 8 weight percent. Additionally, calcium oxide (CaO) and magnesium oxide (MgO), are preferably present in the glass compositions and resulting glass fibers of the present invention in the range of from about 1 weight percent to about 6 weight percent and from about 0 weight percent to about 3.5 weight percent, respectively. The glass fiber compositions of the present invention also include from about 2.0 to about 6.0 weight percent zinc oxide (ZnO), preferably between 2.0 and 5.0 weight percent, from about 0.1 to about 1.5 weight percent F2, preferably between 0.1 to 1.0 weight percent, and very low concentrations of manganese oxide (MnO), ferric oxide (Fe2O3) and possibly various impurities such as strontium oxide (SrO), lithium oxide (Li2O), titanium oxide (TiO2), and zirconium oxide (ZrO2). Preferably, the balance of the composition is silicon dioxide (SiO2). These combinations of components yield the glass compositions of the present invention. Further, the glass compositions of the present invention have suitable viscosities for production of fine-diameter glass fibers using conventional methods, e.g., flame attenuation, rotary spinning, and CAT processes. The glass fibers of the present invention have superior humidity resistance. Accordingly, the glass fibers produced from the compositions of the present invention are well suited for forming HEPA and ULPA glass fiber filters for use in clean room environments.